1. Field of the Invention
The present invention relates to a semiconductor device providing an overvoltage protection function for a switching element.
2. Description of the Related Art
FIG. 4 shows a conventional example of a semiconductor device providing an overvoltage protection function for a switching element.
In FIG. 4, the reference numeral 1 designates a semiconductor switching element, for example, an insulated gate bipolar transistor (IGBT) and the reference numeral 2 designates a free-wheeling diode (FWD). Between the collector C and the gate G of the switching element 1 is connected an overvoltage protection circuit 23 composed of a series-connected circuit of a constant voltage diode 3 and a reverse-blocking diode 5 for a gate current. A constant voltage diode 9 is connected between the gate G and the emitter E of the switching element 1. The constant voltage diode 9 prevents the overvoltage noise due to a gate signal and the overvoltage protection circuit 23 from exceeding the withstand voltage of the gate G of the switching element 1.
The collector C is connected to the positive terminal of the DC power supply and the emitter E is connected to the negative terminal N of the DC power supply.
The reference numeral 8 designates a gate signal output circuit. Between the output terminal of the gate signal output circuit and the gate G of the switching element 1 connected is a series-connected circuit of a gate resistance 7 and a diode 22 for a gate bypass at an OFF time. A gate resistor 6 is connected in parallel with the series-connected circuit of the gate resistor 7 and the diode 22.
When switching drive is conducted on an inductive load (not shown in the figure) by applying a voltage between the collector and the emitter of the switching element 1 in this circuit, a surge voltage is generated due to the collector current variation di/dt at the time of turning OFF of the switching element 1 and the existence of a wiring inductance. If the maximum voltage, which is a voltage between the collector and the emitter superimposed with the surge voltage, exceeds a breakdown voltage between the collector and the emitter, the switching element 1 may breakdown due to overvoltage.
To cope with this issue, in the circuit of FIG. 4, when the voltage at the collector C of the switching element 1 rises to the voltage set by the constant voltage diode 3, a Zener current flows through the diode 3 and raises the gate voltage of the switching element 1 above the ON threshold value even though in the period of gate signal OFF (the period of the gate voltage lower than the ON threshold value). As a result, an electric current flows between the collector and the emitter of the switching element 1 to lower the collector voltage. Thus, the switching element 1 is protected from electric breakdown due to overvoltage.
Patent Document 1 (identified below) discloses an overvoltage protection circuit comprising a clamp circuit including a constant voltage diode between the collector and the gate of an IGBT. When an overvoltage is applied between the collector and the emitter, a Zener current flows through the clamp circuit to turn ON the IGBT in a non-saturated state. Thus, the voltage between the collector and the emitter decreases, performing overvoltage protection.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. H07-297358 (paragraphs [0065], [0066], and FIGS. 1 and 3, in particular)
When an overvoltage is applied between the collector and the emitter of a switching element 1 in the circuit of FIG. 4 or of the Patent Document 1, the switching element 1 is turned ON in a non-saturated state to decrease the voltage between the collector and the emitter. The energy of the surge voltage is self consumed and converted to heat. Although these conventional technologies can avoid electrical breakdown due to overvoltage, the self-heating abruptly increases at the turn OFF time presenting a problem of great burden on the switching element 1 and a gate control circuit for it.